Liquids & Intermolecular Forces
11.4 Surface Tension of Water
Subjects: Properties of liquids, surface tension
Description: A paper clip floats on water. Dish soap is added and the paper clip sinks.
- 600 mL Beaker
- Paper clip
- Dish soap in dropper bottle
- Sulfur (optional –located on general chemical storage shelf) or pepper
Note: Display the demo using the camera
- Fill the beaker with water
- Carefully place a paperclip flat on the surface of the water. (Optional: Sprinkle sulfur or pepper on top of the water surface)
- Touch the surface of the water with your finger and note that the paperclip still doesn’t sink.
- Add a drop of dish soap. The paperclip will sink.
Molecules at the surface of the liquid are not entirely surrounded by other molecules like the molecules beneath the surface are. This leads to a net inward force of attraction, contracting the surface area and making the surface act like a “skin”. Surface tension is a measure of the energy needed to break this tension. The surface tension on the water is enough to hold the paperclip and keep it from breaking the surface. Adding a drop of liquid detergent provides enough energy to break the surface tension of the water, allowing the paper clip to sink.
The solution can be washed down the sink.
1. L. Summerlin, J. Ealy; Chemical Demonstrations: A Sourcebook for Teachers; Volume 1; 1985; p. 135 (sulfur variation)
2. L. Summerlin, C. Borgford, J. Ealy; Chemical Demonstrations: A Sourcebook for Teachers; Volume 2; 1987; p. 19 (needle version)
3. Kotz, Treichel, Townsend; Chemistry & Chemical Reactivity; 7th Edition; Instructor’s Edition; Brooks/Cole; P. 578
4. B.Z. Shakhashiri; Chemical Demonstrations: A Handbook for Teachers of Chemistry; Volume 3; Wisconsin; 1989; p. 301-304 (Procedure B variation)